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// Copyright 2017 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// Package cache implements a build artifact cache.
package cache
import (
"bytes"
"crypto/sha256"
"encoding/hex"
"errors"
"fmt"
"io"
"io/ioutil"
"os"
"path/filepath"
"strconv"
"strings"
"time"
"cmd/go/internal/renameio"
)
// An ActionID is a cache action key, the hash of a complete description of a
// repeatable computation (command line, environment variables,
// input file contents, executable contents).
type ActionID [HashSize]byte
// An OutputID is a cache output key, the hash of an output of a computation.
type OutputID [HashSize]byte
// A Cache is a package cache, backed by a file system directory tree.
type Cache struct {
dir string
now func() time.Time
}
// Open opens and returns the cache in the given directory.
//
// It is safe for multiple processes on a single machine to use the
// same cache directory in a local file system simultaneously.
// They will coordinate using operating system file locks and may
// duplicate effort but will not corrupt the cache.
//
// However, it is NOT safe for multiple processes on different machines
// to share a cache directory (for example, if the directory were stored
// in a network file system). File locking is notoriously unreliable in
// network file systems and may not suffice to protect the cache.
//
func Open(dir string) (*Cache, error) {
info, err := os.Stat(dir)
if err != nil {
return nil, err
}
if !info.IsDir() {
return nil, &os.PathError{Op: "open", Path: dir, Err: fmt.Errorf("not a directory")}
}
for i := 0; i < 256; i++ {
name := filepath.Join(dir, fmt.Sprintf("%02x", i))
if err := os.MkdirAll(name, 0777); err != nil {
return nil, err
}
}
c := &Cache{
dir: dir,
now: time.Now,
}
return c, nil
}
// fileName returns the name of the file corresponding to the given id.
func (c *Cache) fileName(id [HashSize]byte, key string) string {
return filepath.Join(c.dir, fmt.Sprintf("%02x", id[0]), fmt.Sprintf("%x", id)+"-"+key)
}
// An entryNotFoundError indicates that a cache entry was not found, with an
// optional underlying reason.
type entryNotFoundError struct {
Err error
}
func (e *entryNotFoundError) Error() string {
if e.Err == nil {
return "cache entry not found"
}
return fmt.Sprintf("cache entry not found: %v", e.Err)
}
func (e *entryNotFoundError) Unwrap() error {
return e.Err
}
const (
// action entry file is "v1 <hex id> <hex out> <decimal size space-padded to 20 bytes> <unixnano space-padded to 20 bytes>\n"
hexSize = HashSize * 2
entrySize = 2 + 1 + hexSize + 1 + hexSize + 1 + 20 + 1 + 20 + 1
)
// verify controls whether to run the cache in verify mode.
// In verify mode, the cache always returns errMissing from Get
// but then double-checks in Put that the data being written
// exactly matches any existing entry. This provides an easy
// way to detect program behavior that would have been different
// had the cache entry been returned from Get.
//
// verify is enabled by setting the environment variable
// GODEBUG=gocacheverify=1.
var verify = false
var errVerifyMode = errors.New("gocacheverify=1")
// DebugTest is set when GODEBUG=gocachetest=1 is in the environment.
var DebugTest = false
func init() { initEnv() }
func initEnv() {
verify = false
debugHash = false
debug := strings.Split(os.Getenv("GODEBUG"), ",")
for _, f := range debug {
if f == "gocacheverify=1" {
verify = true
}
if f == "gocachehash=1" {
debugHash = true
}
if f == "gocachetest=1" {
DebugTest = true
}
}
}
// Get looks up the action ID in the cache,
// returning the corresponding output ID and file size, if any.
// Note that finding an output ID does not guarantee that the
// saved file for that output ID is still available.
func (c *Cache) Get(id ActionID) (Entry, error) {
if verify {
return Entry{}, &entryNotFoundError{Err: errVerifyMode}
}
return c.get(id)
}
type Entry struct {
OutputID OutputID
Size int64
Time time.Time
}
// get is Get but does not respect verify mode, so that Put can use it.
func (c *Cache) get(id ActionID) (Entry, error) {
missing := func(reason error) (Entry, error) {
return Entry{}, &entryNotFoundError{Err: reason}
}
f, err := os.Open(c.fileName(id, "a"))
if err != nil {
return missing(err)
}
defer f.Close()
entry := make([]byte, entrySize+1) // +1 to detect whether f is too long
if n, err := io.ReadFull(f, entry); n > entrySize {
return missing(errors.New("too long"))
} else if err != io.ErrUnexpectedEOF {
if err == io.EOF {
return missing(errors.New("file is empty"))
}
return missing(err)
} else if n < entrySize {
return missing(errors.New("entry file incomplete"))
}
if entry[0] != 'v' || entry[1] != '1' || entry[2] != ' ' || entry[3+hexSize] != ' ' || entry[3+hexSize+1+hexSize] != ' ' || entry[3+hexSize+1+hexSize+1+20] != ' ' || entry[entrySize-1] != '\n' {
return missing(errors.New("invalid header"))
}
eid, entry := entry[3:3+hexSize], entry[3+hexSize:]
eout, entry := entry[1:1+hexSize], entry[1+hexSize:]
esize, entry := entry[1:1+20], entry[1+20:]
etime, entry := entry[1:1+20], entry[1+20:]
var buf [HashSize]byte
if _, err := hex.Decode(buf[:], eid); err != nil {
return missing(fmt.Errorf("decoding ID: %v", err))
} else if buf != id {
return missing(errors.New("mismatched ID"))
}
if _, err := hex.Decode(buf[:], eout); err != nil {
return missing(fmt.Errorf("decoding output ID: %v", err))
}
i := 0
for i < len(esize) && esize[i] == ' ' {
i++
}
size, err := strconv.ParseInt(string(esize[i:]), 10, 64)
if err != nil {
return missing(fmt.Errorf("parsing size: %v", err))
} else if size < 0 {
return missing(errors.New("negative size"))
}
i = 0
for i < len(etime) && etime[i] == ' ' {
i++
}
tm, err := strconv.ParseInt(string(etime[i:]), 10, 64)
if err != nil {
return missing(fmt.Errorf("parsing timestamp: %v", err))
} else if tm < 0 {
return missing(errors.New("negative timestamp"))
}
c.used(c.fileName(id, "a"))
return Entry{buf, size, time.Unix(0, tm)}, nil
}
// GetFile looks up the action ID in the cache and returns
// the name of the corresponding data file.
func (c *Cache) GetFile(id ActionID) (file string, entry Entry, err error) {
entry, err = c.Get(id)
if err != nil {
return "", Entry{}, err
}
file = c.OutputFile(entry.OutputID)
info, err := os.Stat(file)
if err != nil {
return "", Entry{}, &entryNotFoundError{Err: err}
}
if info.Size() != entry.Size {
return "", Entry{}, &entryNotFoundError{Err: errors.New("file incomplete")}
}
return file, entry, nil
}
// GetBytes looks up the action ID in the cache and returns
// the corresponding output bytes.
// GetBytes should only be used for data that can be expected to fit in memory.
func (c *Cache) GetBytes(id ActionID) ([]byte, Entry, error) {
entry, err := c.Get(id)
if err != nil {
return nil, entry, err
}
data, _ := ioutil.ReadFile(c.OutputFile(entry.OutputID))
if sha256.Sum256(data) != entry.OutputID {
return nil, entry, &entryNotFoundError{Err: errors.New("bad checksum")}
}
return data, entry, nil
}
// OutputFile returns the name of the cache file storing output with the given OutputID.
func (c *Cache) OutputFile(out OutputID) string {
file := c.fileName(out, "d")
c.used(file)
return file
}
// Time constants for cache expiration.
//
// We set the mtime on a cache file on each use, but at most one per mtimeInterval (1 hour),
// to avoid causing many unnecessary inode updates. The mtimes therefore
// roughly reflect "time of last use" but may in fact be older by at most an hour.
//
// We scan the cache for entries to delete at most once per trimInterval (1 day).
//
// When we do scan the cache, we delete entries that have not been used for
// at least trimLimit (5 days). Statistics gathered from a month of usage by
// Go developers found that essentially all reuse of cached entries happened
// within 5 days of the previous reuse. See golang.org/issue/22990.
const (
mtimeInterval = 1 * time.Hour
trimInterval = 24 * time.Hour
trimLimit = 5 * 24 * time.Hour
)
// used makes a best-effort attempt to update mtime on file,
// so that mtime reflects cache access time.
//
// Because the reflection only needs to be approximate,
// and to reduce the amount of disk activity caused by using
// cache entries, used only updates the mtime if the current
// mtime is more than an hour old. This heuristic eliminates
// nearly all of the mtime updates that would otherwise happen,
// while still keeping the mtimes useful for cache trimming.
func (c *Cache) used(file string) {
info, err := os.Stat(file)
if err == nil && c.now().Sub(info.ModTime()) < mtimeInterval {
return
}
os.Chtimes(file, c.now(), c.now())
}
// Trim removes old cache entries that are likely not to be reused.
func (c *Cache) Trim() {
now := c.now()
// We maintain in dir/trim.txt the time of the last completed cache trim.
// If the cache has been trimmed recently enough, do nothing.
// This is the common case.
data, _ := renameio.ReadFile(filepath.Join(c.dir, "trim.txt"))
t, err := strconv.ParseInt(strings.TrimSpace(string(data)), 10, 64)
if err == nil && now.Sub(time.Unix(t, 0)) < trimInterval {
return
}
// Trim each of the 256 subdirectories.
// We subtract an additional mtimeInterval
// to account for the imprecision of our "last used" mtimes.
cutoff := now.Add(-trimLimit - mtimeInterval)
for i := 0; i < 256; i++ {
subdir := filepath.Join(c.dir, fmt.Sprintf("%02x", i))
c.trimSubdir(subdir, cutoff)
}
// Ignore errors from here: if we don't write the complete timestamp, the
// cache will appear older than it is, and we'll trim it again next time.
renameio.WriteFile(filepath.Join(c.dir, "trim.txt"), []byte(fmt.Sprintf("%d", now.Unix())), 0666)
}
// trimSubdir trims a single cache subdirectory.
func (c *Cache) trimSubdir(subdir string, cutoff time.Time) {
// Read all directory entries from subdir before removing
// any files, in case removing files invalidates the file offset
// in the directory scan. Also, ignore error from f.Readdirnames,
// because we don't care about reporting the error and we still
// want to process any entries found before the error.
f, err := os.Open(subdir)
if err != nil {
return
}
names, _ := f.Readdirnames(-1)
f.Close()
for _, name := range names {
// Remove only cache entries (xxxx-a and xxxx-d).
if !strings.HasSuffix(name, "-a") && !strings.HasSuffix(name, "-d") {
continue
}
entry := filepath.Join(subdir, name)
info, err := os.Stat(entry)
if err == nil && info.ModTime().Before(cutoff) {
os.Remove(entry)
}
}
}
// putIndexEntry adds an entry to the cache recording that executing the action
// with the given id produces an output with the given output id (hash) and size.
func (c *Cache) putIndexEntry(id ActionID, out OutputID, size int64, allowVerify bool) error {
// Note: We expect that for one reason or another it may happen
// that repeating an action produces a different output hash
// (for example, if the output contains a time stamp or temp dir name).
// While not ideal, this is also not a correctness problem, so we
// don't make a big deal about it. In particular, we leave the action
// cache entries writable specifically so that they can be overwritten.
//
// Setting GODEBUG=gocacheverify=1 does make a big deal:
// in verify mode we are double-checking that the cache entries
// are entirely reproducible. As just noted, this may be unrealistic
// in some cases but the check is also useful for shaking out real bugs.
entry := fmt.Sprintf("v1 %x %x %20d %20d\n", id, out, size, time.Now().UnixNano())
if verify && allowVerify {
old, err := c.get(id)
if err == nil && (old.OutputID != out || old.Size != size) {
// panic to show stack trace, so we can see what code is generating this cache entry.
msg := fmt.Sprintf("go: internal cache error: cache verify failed: id=%x changed:<<<\n%s\n>>>\nold: %x %d\nnew: %x %d", id, reverseHash(id), out, size, old.OutputID, old.Size)
panic(msg)
}
}
file := c.fileName(id, "a")
// Copy file to cache directory.
mode := os.O_WRONLY | os.O_CREATE
f, err := os.OpenFile(file, mode, 0666)
if err != nil {
return err
}
_, err = f.WriteString(entry)
if err == nil {
// Truncate the file only *after* writing it.
// (This should be a no-op, but truncate just in case of previous corruption.)
//
// This differs from ioutil.WriteFile, which truncates to 0 *before* writing
// via os.O_TRUNC. Truncating only after writing ensures that a second write
// of the same content to the same file is idempotent, and does not — even
// temporarily! — undo the effect of the first write.
err = f.Truncate(int64(len(entry)))
}
if closeErr := f.Close(); err == nil {
err = closeErr
}
if err != nil {
// TODO(bcmills): This Remove potentially races with another go command writing to file.
// Can we eliminate it?
os.Remove(file)
return err
}
os.Chtimes(file, c.now(), c.now()) // mainly for tests
return nil
}
// Put stores the given output in the cache as the output for the action ID.
// It may read file twice. The content of file must not change between the two passes.
func (c *Cache) Put(id ActionID, file io.ReadSeeker) (OutputID, int64, error) {
return c.put(id, file, true)
}
// PutNoVerify is like Put but disables the verify check
// when GODEBUG=goverifycache=1 is set.
// It is meant for data that is OK to cache but that we expect to vary slightly from run to run,
// like test output containing times and the like.
func (c *Cache) PutNoVerify(id ActionID, file io.ReadSeeker) (OutputID, int64, error) {
return c.put(id, file, false)
}
func (c *Cache) put(id ActionID, file io.ReadSeeker, allowVerify bool) (OutputID, int64, error) {
// Compute output ID.
h := sha256.New()
if _, err := file.Seek(0, 0); err != nil {
return OutputID{}, 0, err
}
size, err := io.Copy(h, file)
if err != nil {
return OutputID{}, 0, err
}
var out OutputID
h.Sum(out[:0])
// Copy to cached output file (if not already present).
if err := c.copyFile(file, out, size); err != nil {
return out, size, err
}
// Add to cache index.
return out, size, c.putIndexEntry(id, out, size, allowVerify)
}
// PutBytes stores the given bytes in the cache as the output for the action ID.
func (c *Cache) PutBytes(id ActionID, data []byte) error {
_, _, err := c.Put(id, bytes.NewReader(data))
return err
}
// copyFile copies file into the cache, expecting it to have the given
// output ID and size, if that file is not present already.
func (c *Cache) copyFile(file io.ReadSeeker, out OutputID, size int64) error {
name := c.fileName(out, "d")
info, err := os.Stat(name)
if err == nil && info.Size() == size {
// Check hash.
if f, err := os.Open(name); err == nil {
h := sha256.New()
io.Copy(h, f)
f.Close()
var out2 OutputID
h.Sum(out2[:0])
if out == out2 {
return nil
}
}
// Hash did not match. Fall through and rewrite file.
}
// Copy file to cache directory.
mode := os.O_RDWR | os.O_CREATE
if err == nil && info.Size() > size { // shouldn't happen but fix in case
mode |= os.O_TRUNC
}
f, err := os.OpenFile(name, mode, 0666)
if err != nil {
return err
}
defer f.Close()
if size == 0 {
// File now exists with correct size.
// Only one possible zero-length file, so contents are OK too.
// Early return here makes sure there's a "last byte" for code below.
return nil
}
// From here on, if any of the I/O writing the file fails,
// we make a best-effort attempt to truncate the file f
// before returning, to avoid leaving bad bytes in the file.
// Copy file to f, but also into h to double-check hash.
if _, err := file.Seek(0, 0); err != nil {
f.Truncate(0)
return err
}
h := sha256.New()
w := io.MultiWriter(f, h)
if _, err := io.CopyN(w, file, size-1); err != nil {
f.Truncate(0)
return err
}
// Check last byte before writing it; writing it will make the size match
// what other processes expect to find and might cause them to start
// using the file.
buf := make([]byte, 1)
if _, err := file.Read(buf); err != nil {
f.Truncate(0)
return err
}
h.Write(buf)
sum := h.Sum(nil)
if !bytes.Equal(sum, out[:]) {
f.Truncate(0)
return fmt.Errorf("file content changed underfoot")
}
// Commit cache file entry.
if _, err := f.Write(buf); err != nil {
f.Truncate(0)
return err
}
if err := f.Close(); err != nil {
// Data might not have been written,
// but file may look like it is the right size.
// To be extra careful, remove cached file.
os.Remove(name)
return err
}
os.Chtimes(name, c.now(), c.now()) // mainly for tests
return nil
}